Don't use instance variables in partials

* It is hard to reason about the source of instance variables in
 partials because the source is too far away.
* Like globals, they're not given to you by your caller, and
 maybe not even your caller's caller.
* That makes it hard to figure out what you're changing when you change the
 instance variable.
* It couples the partial to the place it's first used.
* If you use locals, then you can follow the chain all the way back.

Thanks to @gabebw, @calebthompson, and @pbrisbin.